Mountain off-grid cabin

JimPack · July 2017

I'm building a large (4000 sq ft) cabin in Utah at 7600' elevation. I have a shed with a 40' x 20' South facing roof that is 400' from the cabin site. I will have 40kwh of lithium ion batteries (made from 5600 18650 cells) that will be configured at 48volts. In addition to the cabin (120v ac, it will need to run a 2hp/230v ac submersible water pump occasionally (to fill the 2000 gal water tank).
I plan on having a 5kw or 10kw propane generator tied to the system as well for supplemental charging of the batteries as needed.

Lots of questions but for starters: Where should I build the battery storage shed, and where should I put the inverters? Near the house? Near the shed/panels? And why?
(The battery shed will be mostly underground/side_of_a_hill and should be able to maintain a temperature of 50F during the winter and 70F during the summer).

Suggestions?
Jim

Estragon · July 2017

As a rule, you want to keep lower voltage wire runs short, and run longer distances at higher voltages. The reason for this is that for a given wattage, lower current and higher voltage means fewer wire losses (and/or smaller wire).

There are a wide variety of charge controllers that run5 up to about 150v, which works out well for strings of 3 260ish watt pv panels. Going that route, it would probably make sense to put batteries and inverter(s) close to the panels, assuming you have no DC loads aside from inverters. From the inverters, you could run 240v ac to the house panel and the pump.

I would put the electronics in the battery shed for two reasons; first, to keep the ~48v circuits to charge controller and inverters short, and second to keep them cooler in summer. Electronics hate heat.

Solray · July 2017

Are you using logs to build it? I am sure we would all like to see some pictures of it.

Horsefly · July 2017

5600 18650 Li cells?! Wow. That's a ton of cells to keep balanced, and lots of challenge to not get them over-charged, which can be real bad. There are several Lithium experts here that may chime in on what to watch out for. I'm mostly hoping you keep us posted on how it works out.

nickdearing88 · July 2017

+1 on the pictures. I'd also like to hear details on your battery bank plans -- very interested. Did you make your own design with the 18650's or use someone's plans? What are you doing for BMS?

mcgivor · July 2017

@JimPack asks. Where should I build the battery storage shed, and where should I put the inverters? Near the house? Near the shed/panels? And why?

As far away from where you sleep/live for the batteries at least, why? .......fire comes to mind.

JimPack · July 2017

Estragon,
Your recommendation makes sense. I understand "high voltage for distance", and was feeling somewhat uncomfortable with that much wire running high DC volts. Sending AC volts for the long distance run sounds smarter.

I don't plan on any significant DC loads (except for portable device charging, etc).

Regarding the lithium ion batteries, I'll be fussing each cell and using a bms by Batrium (that also offers remote cell monitoring). You can check out a bunch of YouTube videos put out by HBPowerwall for one person's experience (and the concept I'm using). It's a poor man's Tesla powerwall at 1/4 the cost (but much more time commitment). I've got the needed cells, but still need to finish testing and then building.

The batteries will be in a concrete shed with 3 sides and the top underground (for 4 season temperature control and safety).

The cabin will be ICF with log-looking concrete siding and cultured stone (maintenance free and good thermal mass. Something's like you'll see if you Google "Everlog").

I think I can get about 4 hours of shade-free sun on the 40'x20' shed roof (trees and mountains get in the way. Did an asmuth test and that's what I come up with), but the roof angle won't be ideal in the winter (it's also likely to be covered with snow as we get about 6-7' up there). So, good chance, most winter power will come from the generator. We expect to use e cabin only a couple times during the winter, so the power will be primarily just to keep it from freezing (propane hydronic radiant heat) and to keep the monitors (Internet, security, batteries, etc) running.

I've been looking at the Outback line of inverters and other toys. They appear to play well with lithium ion. Is that the experience of this forum? Are there other brands that I should be considering?

Thanks!
Jim

mike95490 · July 2017

JimPack said:

Estragon,
Your recommendation makes sense. I understand "high voltage for distance", and was feeling somewhat uncomfortable with that much wire running high DC volts. Sending AC volts for the long distance run sounds smarter.

I don't plan on any significant DC loads (except for portable device charging, etc).

Regarding the lithium ion batteries, I'll be fussing each cell and using a bms by Batrium (that also offers remote cell monitoring). You can check out a bunch of YouTube videos put out by HBPowerwall for one person's experience (and the concept I'm using). It's a poor man's Tesla powerwall at 1/4 the cost (but much more time commitment). I've got the needed cells, but still need to finish testing and then building.

The batteries will be in a concrete shed with 3 sides and the top underground (for 4 season temperature control and safety).

The cabin will be ICF with log-looking concrete siding and cultured stone (maintenance free and good thermal mass. Something's like you'll see if you Google "Everlog").

I think I can get about 4 hours of shade-free sun on the 40'x20' shed roof (trees and mountains get in the way. Did an asmuth test and that's what I come up with), but the roof angle won't be ideal in the winter (it's also likely to be covered with snow as we get about 6-7' up there). So, good chance, most winter power will come from the generator. We expect to use e cabin only a couple times during the winter, so the power will be primarily just to keep it from freezing (propane hydronic radiant heat) and to keep the monitors (Internet, security, batteries, etc) running.

I've been looking at the Outback line of inverters and other toys. They appear to play well with lithium ion. Is that the experience of this forum? Are there other brands that I should be considering?

Thanks!
Jim

5,600 cell interconnects and fuses are a deal breaker, please rethink this, just because a robotic factory can produce and test that size of battery array, does not scale to a person doing it.
Have you DONE the thermal calculations for keeping your battery room above freezing to prevent destruction of the LIon cells from cold charging?
Your winter monitor loads may seem insignificant to you, but have you accurately added them up and noted your daily idle power requirement. I'd guess it's at least 1Kw because most of all the gear you describe has no energy consciousness,
Any of the major names in off-grid (Schneider, Outback, Morningstar, Midnight) will be suited to LIon, as they all have adjustable setpoints.
Relying on an unattended auto-start generator in winter is a great idea, but bad practice.
Look at some ideas of making a PV array more likely to shed winter snow, more successful. (avoid roof mount and the ice dam problem, space between rows to allow snow to fall, not just slide over the next panel, enough height to allow shed snow to not block array)

Solray · July 2017

Doing a thing is the best way to learn about it.

JimPack · July 2017

Fuse wire is really ideal for 5600 fuses, but if you have a better idea, I'm open to it.
Check out the setup made by HBPowerwall on YouTube.
There are lots of things you learn by doing. It's amazing how much I leaned about flying and airplanes by building one and then flying it across the country.
Regarding winter loads, I am considering two electric panels. One for when I'm away, the other for when I'm there. That will help to ensure minimal load.
There panels are on an unheated shed, so ice dams won't be a problem. With 6' of snow fall, I wonder if it would be best to let it fall completely off, than stagger the panels, however, maybe a "winter tilt angler" might be a good thing to try and configure. It will require some significant hardware, but might be worth the effort. Thanks for the suggestion.
What makes an auto start generator "bad practice".
I've got a test battery shed that is not completely buried, but has kept very temperate. I will test and record the winter temperatures this winter, prior to installing the batteries and electronics next summer. So, I should have much better than a thermal calculation (which would be inherently inaccurate due to the mostly underground nature of the shed). I will definitely keep the lithium ion batteries well above freezing when charging/discharging.
Thanks for the good suggestion of making the array more about to shed snow. What angle so you think would be required to do that? Also, maybe Wash and coat the surface in the fall with something like RainX?
Thanks,
Jim

Solray · July 2017

I have always wanted to do an around the world flight in my own plane, but have yet to do it. Good for you.
I wouldn't use rain-x on a panel, maybe polish the glass nicely though.

Estragon · July 2017

I would design the water system to be drained down and blown out with air, even if you plan to heat the cabin. I do that at my city house while I'm south in winter as the insurance is void if pipes burst in an unattended house unless it's checked frequently.

The heat may be propane, but I assume circ pumps are electric? Mine use around 200w for ~1000' of 1/2" radiant pipe. I just use it to take the chill off the floors, as the woodstove heat stratifies and leaves floors cold. Running for an hour or two isn't bad, but 24/7 would be a major load.

The panel angle for snow depends on what kind of snow. My panels are at 65° for winter, which is fine for cold, dry snow. The last couple of years though, we had milder weather in late fall/early winter, so wet snow stuck to the array and hardened on when it got colder and they would have stayed completely covered until spring if I hadn't cleared them manually.

JimPack · July 2017

Ok. So we're saying:
- Batteries and inverter near the solar panels.
- Confirm that the mostly underground battery bunker doesn't freezer during the winter.
- Run 240 volts from the inverters to the electric breaker box at the house, which is 300'-400' away. (Need to determine the wire guage).
- Make the frame that the solar panels sit on adjustable to 65° or more for winter (for best asmuth angle and snow shedding).
- Configure one of the breaker panels to be an "always on" panel for when we are not at the cabin, and do a detailed analysis of the away-load.
- Propane powered generator (size TBD), tired into the battery bank to recharge then when battery levels drop below X% (I'm thinking 25%??).
- 40 kwh lithium battery bank, operating at 48 v dc, with active BMS designed for that size bank. (Each cell is expected to be 740 watt hours @ 3.7 volts, 200 amp-hours). 4 banks of 14 cells in series for a total of 10.36 kwh each bank.
- need to determine total load on days we are at the cabin.
- how much juice can be produced with a 20'x40' roof? Will it be enough to keep the generator from running to much?

myocardia · July 2017

JimPack said:
- how much juice can be produced with a 20'x40' roof? Will it be enough to keep the generator from running to much?

That depends 100% on how much power you use. You can easily use 10x the amount that amount of roof space can produce, and you can also easily use 1/8 that amount, or even less. The amount that you use is completely up to you and the other people who are there, along with how efficiently the place has been designed..

JimPack · July 2017

True enough! I'll need to do some figuring.

Dave Angelini · July 2017

I use hi-voltage DC from the array to the house where I would locate the battery/inverter. Even on an outside insulated wall where you can easily look at the system. Especially in snow country. I dislike sheds immensely for the reason that you will be too far from the battery.
I would stay with a system completely made from Outback or Schneider Solar. Mount solar on the ground and save the roof for what it is meant to do Offgrid. Check your surge on that 2HP motor, if it is a slow start you are fine but the full surge on Lithium is not a sustainable practice. Lead Acid is much better for surge performance. Good Luck

jonr · July 2017

Might even be worth it to replace the well pump with a much smaller (perhaps 1/2 HP) one. Or always use the generator to run the well pump.

A well insulated lithium battery box should require very little energy to maintain 50F.

I find it interesting that Tesla will be using lots of small lithium cells (21700, a little bigger than 18650).

mike95490 · July 2017

+5,000 cell interconnects, knock yourself out keeping them all working.

Backup propane genset. Will someone check it weekly, monthly, when you return in the spring ?
Many things glitch a autostart genset. propane valve sticks, and no gas flow, I'm sure there is a time-out on the starter motor circuit, but how is a fault detected when it happens ? Oil level sensor sticks (like mine started doing) and genset never starts because it thinks low oil. or a ring breaks in the cylinder. All the moving parts and oil issues can shut it down. Is the propane tank large enough to not freeze up as you draw off fuel ?

Solray · July 2017

It sounds like you think that because a machine has the potential to break, we should avoid using any machines at all.
With a little common sense and an understanding of how things work, steps can easily be taken to avoid most if not all of the problems associated with the system he is proposing. Others have done it successfully so it must be possible. There is always a chance of a failure bringing down any mechanized system, but these can be brought to such a low level that they are never seen in operation if it's handled well.
Because something can fail is not a reason not to try it. We would never accomplish anything if everyone shared your doomsayer outlook.

Estragon · July 2017

A 20x40' shed is about 6x12=72 sq metre. Under standard test conditions (cool panels, 1000w/sqm insolation) panels are generally rated at about 14-18% efficient, so output would be on the order of 72x15= about 1kw of output. Most of us figure on about 75% of that, but your altitude and likely cool temps mean you may do better than many.

On the generator start thing, I'm not a fan, but people do it with apparent success. The thing is it will fail for reasons mike mentioned, among others, the only question is when. If you know it will fail, the issue is whether the failure can be mitigated, and what the consequences of the failure are. The bank could be protected with a low voltage cut-off, and lithium are ok with sitting at partial state of charge. Burst pipes from freezing strikes me as more of a problem.

Solray · July 2017

Charge controllers will fail too. So will all the other components in any system. The key is to do maintenance on the mechanical parts suxh as the generator when it is still working so the work is done on your schedule and not wait for a breakdown to happen. Pro active maintenance is a lot better that reactive maintenance work.

Horsefly · July 2017

@Solray, you seem to know alot about this. What sort of preventative maintenance would you suggest for charge controllers?

mcgivor · July 2017

Rhetorical replies, take them at face value.

Dave Angelini · July 2017

Words of wisdom mcgivor

Horsefly · July 2017

mcgivor said:

Rhetorical replies, take them at face value.

Yeah, that's kinda what I was thinking.

Solray · July 2017

LOL. Charge controllers have no required maintenance schedule, I am surprised you did not know that already. For things that require maintenance, like internal combustion engines, there are several items that need to be adjusted or replaced at certain intervals. If you are not sure, you can inquire with the manufacturer of the product in question.
Some people were saying not to use a generator because it has the possibility of failing but if the maintenance is performed on or before it is scheduled and before leaving it for a period of time, the failures can be virtually eliminated.
I think you got confused as to which item I was referring when mentioning preventative maintenance. If you own a mechanical charge controller with motors and gears, there may be some maintenance required for it. Otherwise, I'd just keep a back up ready to go in case of a failure.

Lumisol · July 2017

Actually, if you own the proper diagnostic hardware and software you can run a diagnostic test on the charge controller to make sure it is within the design parameters and functioning properly. Those are usually only available to repair technicians working for the repair company, however, they may be available through non traditional lines.

littleharbor2 · July 2017

While not a regular maintenance item, some controllers that have fans as part of their cooling would benefit from a semi annual blowing out, especially in a dusty environment.

mike95490 · July 2017

Often, the screw terminals on the copper wires, can benefit from re-checking the torque on the wire/screw. Copper cold flows a bit, and can easily heat a Charge controller terminal block up and damage it.

And I didn't say you can't use a auto-start backup generator, but that they find ways to fail, Better to design to not require it, but have it on standby. Designing / Relying on it running weekly to keep batteries charged can lead to surprises.

https://www.homepower.com/automatic-generator-start
Longtime installer Todd Cory is more blunt: “I never configure autostart generators. Having a machine automatically start/stop while unattended is at best a recipe for damage to gear and, at worst, a fire that could destroy the entire homestead.”

http://idesystems.co.uk/five-reasons-why-generators-fail-to-start-and-what-you-can-do-to-prevent-it/

Look at the reasons they fail, when batteries are low, and the charger is pulling hard from the generator, does the current exceed the generator rating? Battery charging is HIGHLY demanding on a generator. Know and address the problems others have had and you will have fewer,
Common failure - the starting battery has no remote charger, and sitting for 3 months, become too low to crank. Or some generators
have no charging circuit, and you have to rely on a external float charger.

JimPack · July 2017

Estragon. You said, "A 20x40' shed is about 6x12=72 sq metre. Under standard test conditions (cool panels, 1000w/sqm insolation) panels are generally rated at about 14-18% efficient, so output would be on the order of 72x15= about 1kw of output."

I figured on a 20x40' roof I could get 33 or 36 panels. At 300 watts each, that would be closer to 10kw. no?

Estragon · July 2017

Yup, typo. 72,000 x .15 = ~10,000.

Mountain off-grid cabin

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